National Instruments PC-DIO-24/PnP - Manual (Page 64)

Appendix C Register-Level Programming
PC-DIO-24/PnP User Manual C-4 © National Instruments Corporation
Figure C-1. Control Word Formats for the 82C55A
Caution: During programming, note that each time any port is configured, output
ports A and C are reset to 0, and output port B is undefined.
D2 D1 D0 D5 D4 D3 D7 D6
(high nibble)
1 = input
0 = output
Control Word
Flag
1 = mode set
00 = mode 0
01 = mode 1
1X = mode 2 Mode Selection
1 = input
0 = output Port A
Port C
(low nibble)
1 = input
0 = output Port C
0 = mode 0
1 = mode 1 Mode Selection
Group A Group B
Port B
1 = input
0 = output
Flag
D2 D1 D0 D3 D7
0 = bit set/reset
1 = set
0 = reset
(000)
(001)
(010)
:
:
(111)
Control Word
Bit Select
Bit Set/Reset
a. Mode Set Word Format
b. Bit Set/Reset Word Format
Unused
D6 D5 D4
!